This project investigates the functional significance of the feedback pathway to the inner ear, the olivocochlear bundle (OCB). We have long known that electrical stimulation of the OCB raises thresholds in the auditory periphery, when acoustic stimuli are tones in a quiet environment. More recent work has suggested that shock-induced OCB activity might act to decrease thresholds to tones in certain noisy environments. Since OCB activation can be elicited in response to sound, this efferent reflex could function in enhancing signal detection in noise. The aim of this project is to quantify efferent-mediated improvements in signal-to-noise ratios in the response of single auditory nerve fibers. Such basic animal studies are needed to understand the function of the OCB in the human. Malfunction of this feedback system in the hearing impaired may prove to be a significant cause of their well-known difficulties with communication in noisy environments.